Synchronous rectification control circuit and power supply thereof

ABSTRACT

In one embodiment, a synchronous rectification control circuit in a flyback converter, can include: a first control circuit that receives a drain-source voltage signal of a synchronous rectifier switch and a flyback converter output voltage, and generates a first control signal based on a conduction time of a primary-side power switch; a second control circuit configured to receive the drain-source voltage signal, and to generate a second control signal; when the primary-side power switch is turned off, and the first control signal is greater than a threshold value, the second control signal controls a switching operation of the synchronous rectifier switch; and when the primary-side power switch is turned off, and the first control signal is less than the threshold value, the synchronous rectifier switch is configured to stop operation, before the primary-side power switch is turned on again.

RELATED APPLICATIONS

This application is a continuation of the following application, U.S. patent application Ser. No. 13/897,966, filed on May 20, 2013, and which is hereby incorporated by reference as if it is set forth in full in this specification, and which also claims the benefit of Chinese Patent Application No. 201210230736.9, filed on Jul. 2, 2012, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of synchronous rectification, and more specifically to a synchronous rectification control circuit and a switch mode power supply.

BACKGROUND

Flyback converters are widely used due to isolated input-output voltages, relatively low product cost, relatively low space requirements, and other benefits. However, when the output current is relatively high and the output voltage is relatively low, conduction losses and reverse recovery losses of a secondary-side rectifier diode in a traditional flyback converter can be relatively high, resulting in reduced efficiency. In one approach, a MOSFET power transistor with relatively low on resistance can be used in a synchronous rectification technique to replace the rectifier diode, and to improve the conversion efficiency of the converter.

SUMMARY

In one embodiment, a synchronous rectification control circuit in a flyback converter, can include: (i) a first control circuit configured to receive a drain-source voltage signal of a synchronous rectifier switch and an output voltage of the flyback converter, and to generate a first control signal based on a conduction time of a primary-side power switch of the flyback converter, where the first control signal, the conduction time of the primary-side power switch, and a primary-side input voltage of the flyback converter, are directly proportional to each other; (ii) a second control circuit configured to receive the drain-source voltage signal, and to generate a second control signal therefrom; (iii) where when the primary-side power switch is turned off, and the first control signal is greater than a threshold value, the second control signal is configured to control a switching operation of the synchronous rectifier switch before the primary-side power switch is turned on again; and (iv) where when the primary-side power switch is turned off, and the first control signal is less than the threshold value, the synchronous rectifier switch is configured to stop operation before the primary-side power switch is turned on again.

Embodiments of the present invention can advantageously provide several advantages over conventional approaches. For example, particular embodiments can provide a synchronous rectification control circuit and a switch mode power supply that may avoid a secondary-side negative current that might otherwise appear when satisfying a minimum synchronous rectifier switch conduction time. Other advantages of the present invention may become readily apparent from the detailed description of preferred embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an example synchronous rectification control approach.

FIG. 2 is an example operating waveform diagram of the synchronous rectification control approach shown in FIG. 1.

FIG. 3 is a schematic diagram of a first example synchronous rectification control circuit in accordance with embodiments of the present invention.

FIG. 4 is a schematic diagram of a second example synchronous rectification control circuit in accordance with embodiments of the present invention.

FIG. 5 is an example waveform diagram of the synchronous rectification control circuit shown in FIG. 4 under QR mode.

FIG. 6 is an example waveform diagram of the synchronous rectification control circuit shown in FIG. 4 under CCM mode.

FIG. 7 is an example waveform diagram of the synchronous rectification control circuit shown in FIG. 4 under DCM mode.

FIG. 8 is a schematic diagram of an example power supply in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention may be described in conjunction with the preferred embodiments, it may be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set fourth in order to provide a thorough understanding of the present invention. However, it may be readily apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, processes, components, structures, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention.

Shown in FIG. 1 is an example synchronous rectification control approach. When a primary-side of the flyback converter is shut down or turned off, drain-source voltage signal V_(ds) of the synchronous rectifier switch may decrease significantly until it is reduced, e.g., to a potential of several hundreds millivolts lower than zero (e.g., threshold value V_(H1)). Then, control signal Gate may go high to turn on the synchronous rectifier switch. During the rise from a negative voltage to about zero, when drain-source voltage signal V_(ds) reaches a level of predetermined off threshold value V_(H2), control signal Gate may go low to turn off the synchronous rectifier switch.

In order to prevent an inadvertent off condition of the synchronous rectifier switch that may be caused by oscillation after drain-source voltage signal V_(ds) is reduced to threshold value V_(H1), minimum conduction time T_(min,on) can be set. As a result, when drain-source voltage signal V_(ds) reaches a level of a predetermined off threshold value, if the minimum conduction time condition is not satisfied, the synchronous rectifier switch may remain on to keep conducting. As shown in FIG. 2, from time t₁ to time t₂, secondary-side current I_(S) can be reduced to a negative value during a portion of the minimum conduction time due to limited power transmitting capacity of the primary-side. Also, this negative current may result in a reduction of the entire circuit conversion efficiency.

In one approach, the synchronous rectifier switch may be turned off in the next switch period after such a negative current appears. This means that the synchronous rectifier switch can remain off because no Gate signal may be generated from time t₂ to time t₃ in FIG. 2. Though such an approach may prevent the secondary-side from generating this negative current, negative current in secondary-side current I_(S) may not be completely eliminated as the synchronous rectifier switch is controlled to turn off in a next switch period after the appearance of the negative current.

In one embodiment, a synchronous rectification control circuit in a flyback converter, can include: (i) a first control circuit configured to receive a drain-source voltage signal of a synchronous rectifier switch and an output voltage of the flyback converter, and to generate a first control signal based on a conduction time of a primary-side power switch of the flyback converter, where the first control signal, the conduction time of the primary-side power switch, and a primary-side input voltage of the flyback converter, are directly proportional to each other; (ii) a second control circuit configured to receive the drain-source voltage signal, and to generate a second control signal therefrom; (iii) where when the primary-side power switch is turned off, and the first control signal is greater than a threshold value, the second control signal is configured to control a switching operation of the synchronous rectifier switch before the primary-side power switch is turned on again; and (iv) where when the primary-side power switch is turned off, and the first control signal is less than the threshold value, the synchronous rectifier switch is configured to stop operation (e.g., a switching operation of the synchronous rectifier switch) before the primary-side power switch is turned on again.

During the conduction interval of the primary-side power switch, a synchronous rectification control circuit in particular embodiments can determine whether the conduction time of the synchronous rectifier switch in a same switch period can meet a minimum conduction time requirement. The synchronous rectifier switch can then be controlled to turn on or turn off according to this result. In this way, secondary-side negative current can be substantially or totally eliminated to save power, and to improve the circuit conversion efficiency. In addition, particular embodiments may support various switch mode power supply operating modes, such as continuous current mode (CCM), discontinuous current mode (DCM), and quasi-resonance mode (QR).

Referring now to FIG. 3, shown is a schematic diagram of a first example synchronous rectification control circuit in accordance with embodiments of the present invention. For example, a synchronous rectification control circuit of particular embodiments may be used in a flyback converter to control a switching operation of synchronous rectifier switch SR. Of course, the control circuitry of particular embodiments may also be utilized in other types of converters or regulators.

In the particular example of FIG. 3, control circuits 350 and 352 can be employed. Control circuit 350 can receive drain-source voltage signal V_(ds) of synchronous rectifier switch SR, and output voltage V_(o) of the flyback converter. Control circuit 350 can generate control signal V_(ramp) according to a conduction time of primary-side power switch SM of the flyback converter. In addition, control signal V_(ramp), the conduction time of primary-side power switch SM, and primary-side input voltage V_(in) of the flyback converter may be proportional to each other.

Control circuit 352 can receive drain-source voltage signal V_(ds) to generate control signal Gate. When primary-side power switch SM is turned off, and control signal V_(ramp) is higher than threshold value V_(th), control signal Gate can control a switching operation of synchronous rectifier switch SR before primary-side power switch SM is turned on again. When primary-side power switch SM is turned off, and control signal V_(ramp) is less than threshold value V_(th), synchronous rectifier switch SR may stop operation before primary-side power switch SM is turned on again. For example, threshold value V_(th) can be set to a value of a corresponding control signal V_(ramp) with respect to a minimum conduction time of the synchronous rectifier switch SR.

In this particular example, control circuit 350 can include ramp signal generating circuit 301 and enable signal generating circuit 302. Ramp signal generating circuit 301 can receive drain-source voltage signal V_(ds), output voltage V_(o), and clock signal CLK, may generate control signal V_(ramp). For example, control or ramp signal V_(ramp) may be in proportion to a conduction time of primary-side power switch SM and primary-side input voltage V_(in) of the flyback converter.

Enable signal generating circuit 302 can be used to generate an enable signal EN according to control signal V_(ramp) and threshold value V_(th). The state of enable signal EN can be used to determine whether a switching operation of synchronous rectifier switch SR is controlled by control signal Gate. When primary-side power switch SM is turned off, and control signal V_(ramp) is higher than threshold value V_(th), enable signal EN can be active to ensure that control signal Gate can control the switching operation of synchronous rectifier switch SR before primary-side power switch SM is turned on again. When primary-side power switch SM is turned off, and control signal V_(ramp) is less than threshold value V_(th), enable signal EN can be inactive to ensure that synchronous rectifier switch SR stops the switching operation before primary-side power switch SM is turned on again.

In order to achieve the above functions, enable signal generating circuit 302 can include comparison circuit 303, clock signal generating circuit 304, and flip-flop circuit 305. Comparison circuit 303 can be used to compare control signal V_(ramp) and threshold value V_(th) to generate a comparison signal. Clock signal generating circuit 304 can receive drain-source voltage signal V_(ds) and output voltage V_(o) to output clock signal CLK. Flip-flop circuit 305 can receive the clock signal and the comparison signal, and may output enable signal EN at a rising edge of clock signal CLK.

During a conduction interval of primary-side power switch SM, the synchronous rectification control circuit in particular embodiments can determine whether the conduction time of synchronous rectifier switch SR in a same switch period can meet a minimum conduction time requirement. The synchronous rectifier switch can be controlled to turn on or turn off according to this result, thus substantially eliminating secondary-side negative current to save power and to improve circuit conversion efficiency. The implementation of control circuit 352 can be any appropriate circuit structure to generate control signal Gate.

Referring now to FIG. 4, shown is a schematic diagram of a second example constant voltage constant current controller in accordance with embodiments of the present invention. As shown in this particular example, ramp signal generating circuit 301 can include a voltage controlled current source and a charging circuit that includes capacitor C₁ and switch S₁. The input voltage of the voltage controlled current source can be a difference between drain-source voltage signal V_(ds) and output voltage signal V_(o). For example, the output current of the voltage controlled current source may be in direct proportion to the input voltage. Clock signal CLK can be used to control a switching operation of switch S₁ to control charging and discharging operation of the charging circuit. Voltage V_(ramp) (e.g., ramp or control signal) may be output from ramp signal generating circuit 301, and may have a ramp-shape across capacitor C₁.

Comparison circuit or comparator 303 may have a non-inverting input terminal to receive the control signal V_(ramp), and an inverting input terminal to receive threshold value V_(th). Comparison circuit 303 can output a comparison signal to a D-input of flip-flop circuit 305. The clock signal generating circuit 304 can also be a comparator with a non-inverting input terminal receiving output voltage signal V_(o), and an inverting input terminal receiving drain-source voltage signal V_(ds). Comparator or clock signal generating circuit 304 may output clock signal CLK as shown. Flip-flop circuit 305 can include a D-type flip-flop, where the D input terminal can receive the comparison signal from comparator 303, and clock signal CLK can be used to control the output time of the comparison signal to generate enable signal EN at the Q output of flip-flop 305.

When clock signal CLK goes high, switch S₁ may be on to conduct, and ramp control signal V_(ramp) can be discharged. In order to prevent a timing issue of the output signal of comparator 303 being effectively output by flip-flop 305 when V_(ramp) is discharged, delay circuit 402 can be used to delay clock signal CLK that controls the switching operation of switch S₁ by a predetermined delay about DLY.

For example, a synchronous rectification control circuit in particular embodiments can be used in CCM, DCM, or QR operating modes. In this particular example, QR mode may be exemplified when the synchronous rectification control circuit of FIG. 4 is viewed in conjunction with the example operating waveforms of FIG. 5.

For example, k can denote a coefficient of proportionality or a proportional relationship of the output current and the input voltage in the voltage controlled current source. Output current I can be as shown below in Equation 1. I=k(V _(ds) −V _(o))  (1)

During a conduction period of primary-side power switch SM, the drain-source voltage signal of secondary-side synchronous rectifier switch SR may be

${V_{ds} = {\frac{V_{in}}{n} + V_{o}}},$ where n can denote the turns ratio of primary and secondary windings in the flyback converter, and Equation 2 shown below can be obtained. V _(in) =n(V _(ds) −V _(o))  (2)

Deriving from Equations 1 and 2, it can be seen that current I charging capacitor C₁ and primary-side input voltage V_(in) can be in a certain proportion relationship (e.g., in direct proportion). When primary-side power switch SM is conducting, drain-source voltage signal V_(ds) can be higher than output voltage signal V_(o), and clock signal CLK can be low. After a delay time (e.g., via delay circuit 402), switch S₁ can be turned off, and output current I of the voltage controlled current source can begin charging capacitor C₁. Thus, the charging time of capacitor C₁ can approximately be equal to conduction time t_(on) of primary-side power switch SM. In addition, ramp signal V_(ramp) can be denoted by Equation 3, and ramp control signal V_(ramp) can represent a product of primary-side input voltage V_(in) and primary-side power switch SM.

$\begin{matrix} {V_{ramp} = \frac{{k\left( {V_{ds} - V_{o}} \right)}t_{on}}{C_{1}}} & (3) \end{matrix}$

When primary-side power switch SM is conducting, the primary-side current peak

${I_{p - {pk}} = \frac{V_{in} \times t_{on}}{L_{m}}},$ where L_(m) is an equivalent primary-side magnetizing inductance. The secondary-side current peak can be shown as below in Equation 4.

$\begin{matrix} {I_{s - {pk}} = {{n \times I_{p - {pk}}} = {\frac{n \times V_{in} \times t_{on}}{L_{m}} = \frac{\frac{V_{in}}{n} \times t_{on}}{\frac{L_{m}}{n^{2}}}}}} & (4) \end{matrix}$

Thus, conduction time T of secondary-side synchronous rectifier switch SR can be as shown below in Equation 5.

$\begin{matrix} {T = {\frac{I_{s - {pk}}\frac{L_{m}}{n^{2}}}{V_{o}} = \frac{V_{in}t_{on}}{n\; V_{o}}}} & (5) \end{matrix}$

Therefore, it can be concluded that conduction time T of secondary-side synchronous rectifier switch SR can be proportional to the product of primary-side input voltage V_(in) and conduction time t_(on) of primary-side power switch SM. From equations 3 and 5 above, it can be seen that ramp control signal V_(ramp) can represent conduction time T of synchronous rectifier switch SR. also threshold value V_(th) can set to be a value of the ramp control signal V_(ramp) corresponding to a minimum conduction time of the synchronous rectifier switch.

When the primary-side power switch is turned off, whether a negative current will appear (or not) when turning on the synchronous rectifier switch SR before primary-side power switch SM is conducting, can be detected by comparing ramp control signal V_(ramp) against threshold value V_(th).

During the switch period from time t₀ to time t₁ (see, e.g., FIG. 5), when primary-side power switch SM is turned off, and ramp control signal V_(ramp) is greater than threshold value V_(th), can mean that in this switching period, if synchronous rectifier switch SR is on, the conduction time of SR can be longer than the minimum conduction time. Thus, at the moment when primary-side power switch SM is turned off, secondary-side current I_(S) may not become a negative current, so at the turning off time of primary-side power switch SM, drain-source voltage signal V_(ds) may decrease significantly to be less than output voltage signal V_(o). Also, a rising edge of clock signal CLK can occur, the output of comparator 303 can go high, and the output of flip-flop 305 can be configured as enable signal EN. Therefore, along with enable signal EN, control signal Gate can be used to turn on the synchronous rectifier switch via AND-gate 354.

During the switch period from time t₁ to time t₂, at the moment when primary-side power switch SM is turned off, and ramp control signal V_(ramp) is less than threshold value V_(th), can mean that in the switching period, if synchronous rectifier switch SR is on, the conduction time of SR can be less than the minimum conduction time, and the negative current may appear at the secondary-side. Thus, the output of comparator 303 can be low, and correspondingly, enable signal EN may be low to keep synchronous rectifier switch SR off. Secondary-side current I_(S) may decrease to zero due to the function of the body diode of synchronous rectification switch SR to totally avoid the negative current. The operations in the switch period from time t₂ to time t₃ may be similar.

As seen in the particular example of FIG. 4, the synchronous rectification control circuit can control capacitor charging current to follow a difference variation of drain-source voltage signal V_(ds) and output voltage signal V_(o). The charging time of capacitor C₁ can be controlled to be approximately equal to the conduction time of primary-side power switch SM. Also, a ramp control signal V_(ramp) may be in direct proportion to the conduction time of primary-side power switch SM. Further, by comparing ramp control signal V_(ramp) against threshold value signal V_(th) corresponding to the minimum conduction time, whether control signal Gate should be enabled (e.g., via enable signal EN) in the switching period can be determined. In this way, negative secondary-side current (e.g., in the secondary winding) can be substantially eliminated, and the overall circuit conversion efficiency can be improved.

While example specific implementations of the ramp signal generating circuit and the enable signal generating circuit are shown and described herein, those skilled in the art will recognize that other appropriate circuit structures can be utilized in particular embodiments. Also, synchronous rectification control circuits described herein may be utilized with various operation modes (e.g., CCM, DCM, QR, etc.) of various converters (e.g., flyback converter, etc.). For example, FIG. 6 is an example waveform diagram of the synchronous rectification control circuit shown in FIG. 4 under CCM mode, and FIG. 7 is an example waveform diagram of the synchronous rectification control circuit shown in FIG. 4 under DCM mode.

Referring now to FIG. 8, shown is a schematic diagram of an example power supply in accordance with embodiments of the present invention. In this example, the power supply can include a transformer with a primary winding and a secondary winding. The transformer can be used to transmit input voltage V_(in) to the secondary-side. Primary-side power switch SM can be series connected with the primary winding, and primary-side power switch SM can be used to control the primary-side winding to receive power from input voltage V_(in). Primary-side control circuit 802 can be used to control a switching operation of primary-side power switch SM. Synchronous rectifier switch SR can be series connected with the secondary winding. Synchronous rectification control circuit 804 can be used to control the switching operation of synchronous rectifier switch SR to convert received power to a certain output voltage V_(o). The particular implementations can be any suitable synchronous rectification control circuits.

The foregoing descriptions of specific embodiments of the present invention have been presented through images and text for purpose of illustration and description of the synchronous rectification control circuit and associated power supply. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching for different applications.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. 

What is claimed is:
 1. A synchronous rectification control circuit in a flyback converter, comprising: a) a first control circuit configured to receive a drain-source voltage signal of a synchronous rectifier switch and an output voltage of said flyback converter, and to generate a first control signal based on a conduction time of a primary-side power switch of said flyback converter, wherein said first control signal, said conduction time of said primary-side power switch, and a primary-side input voltage of said flyback converter, are directly proportional to each other; b) a second control circuit configured to receive said drain-source voltage signal, and to generate a second control signal therefrom; c) wherein only when said primary-side power switch is turned off, and said first control signal is greater than a threshold value to activate an enable signal, said second control signal is enabled by said enable signal to control a switching operation of said synchronous rectifier switch before said primary-side power switch is turned on again; and d) wherein when said primary-side power switch is turned off, and said first control signal is less than said threshold value, said synchronous rectifier switch is configured to disable operation before said primary-side power switch is turned on again.
 2. The synchronous rectification control circuit of claim 1, wherein said threshold value is substantially equal to a value of said first control signal that corresponds to a minimum conduction time of said synchronous rectifier switch.
 3. The synchronous rectification control circuit of claim 1, wherein said first control circuit comprises: a) an enable signal generating circuit configured to generate said enable signal based on said first control signal and said threshold value signal; b) wherein when said enable signal is active, said second control signal is enabled to control a switching operation of said synchronous rectifier switch; and c) wherein when said enable signal is inactive, said synchronous rectifier switch is configured to disable said switching operation.
 4. The synchronous rectification control circuit of claim 3, wherein said enable signal generating circuit comprises: a) a comparison circuit configured to compare said first control signal against said threshold value, and to generate a comparison signal; b) a clock signal generating circuit configured to receive said drain-source voltage signal and said output voltage, and to generate a clock signal; and c) a flip-flop circuit configured to receive said clock signal and said comparison signal, and to output said comparison signal as said enable signal at a rising edge of said clock signal.
 5. The synchronous rectification control circuit of claim 4, wherein said first control circuit comprises a ramp signal generating circuit configured to receive said drain-source voltage signal, said output voltage, and said clock signal, and to generate a ramp signal as said first control signal.
 6. The synchronous rectification control circuit of claim 5, wherein said ramp signal generating circuit comprises: a) a charging circuit comprising a voltage controlled current source and a first capacitor, wherein an input voltage of said voltage controlled current source comprises a difference between said drain-source voltage signal and said output voltage signal, and wherein an output current of said voltage controlled current source is in direct proportion to said input voltage; and b) a first switch, wherein said clock signal is configured to control a switching operation of said first switch to control charging and discharging operation of said charging circuit, wherein a voltage of said first capacitor is configured as said first control signal.
 7. The synchronous rectification control circuit of claim 6, further comprising a delay circuit, wherein said clock signal is configured to control said switching operation of said first switch after being delayed by said delay circuit.
 8. The synchronous rectification control circuit of claim 5, wherein said comparison circuit comprises a first comparator configured to receive said ramp signal and said threshold value, and to output said comparison signal.
 9. The synchronous rectification control circuit of claim 4, wherein said clock signal generating circuit comprises a second comparator configured to receive said output voltage signal and said drain-source voltage signal, and to output said clock signal.
 10. A switch mode power supply, comprising the synchronous rectification control circuit of claim 1, and further comprising: a) a transformer having a primary winding and a secondary winding, wherein said transformer is configured to transmit an input voltage to a secondary-side; b) said primary-side power switch coupled in series with said primary winding, wherein said primary-side power switch is configured to control said primary winding to receive power from said input voltage; c) a primary-side control circuit configured to control a switching operation of said primary-side power switch; and d) a synchronous rectifier switch coupled in series with said secondary winding, wherein said synchronous rectification control circuit is configured to control a switching operation of said synchronous rectifier switch to convert received electric energy at said secondary-side to a predetermined output voltage.
 11. The switch mode power supply of claim 10, wherein an operation mode of said switch mode power supply comprises at least one of a continuous current mode, a discontinuous current mode, and a quasi-resonance mode.
 12. A method of controlling a flyback converter, the method comprising: a) receiving, by a first control circuit, a drain-source voltage signal of a synchronous rectifier switch and an output voltage of said flyback converter, and generating a first control signal based on a conduction time of a primary-side power switch of said flyback converter, wherein said first control signal, said conduction time of said primary-side power switch, and a primary-side input voltage of said flyback converter, are directly proportional to each other; b) receiving, by a second control circuit, said drain-source voltage signal, and generating a second control signal therefrom; c) only when said primary-side power switch is turned off, and said first control signal is greater than a threshold value to activate an enable signal, enabling said second control signal by said enable signal to control a switching operation of said synchronous rectifier switch before said primary-side power switch is turned on again; and d) when said primary-side power switch is turned off, and said first control signal is less than said threshold value, disabling operation of said synchronous rectifier switch before said primary-side power switch is turned on again.
 13. The method of claim 12, wherein said threshold value is substantially equal to a value of said first control signal that corresponds to a minimum conduction time of said synchronous rectifier switch.
 14. The method of claim 12, further comprising: a) generating, by an enable signal generating circuit, said enable signal based on said first control signal and said threshold value signal; b) when said enable signal is active, enabling said second control signal to control a switching operation of said synchronous rectifier switch; and c) when said enable signal is inactive, disabling said switching operation by said synchronous rectifier switch.
 15. The method of claim 14, wherein said generating said enable signal comprises: a) comparing, by a comparison circuit, said first control signal against said threshold value, and generating a comparison signal therefrom; b) receiving, by a clock signal generating circuit, said drain-source voltage signal and said output voltage, and generating a clock signal therefrom; and c) receiving, by a flip-flop circuit, said clock signal and said comparison signal, and providing said comparison signal as said enable signal at a rising edge of said clock signal.
 16. The method of claim 15, further comprising: a) receiving, by a ramp signal generating circuit, said drain-source voltage signal, said output voltage, and said clock signal; and b) generating a ramp signal as said first control signal.
 17. The method of claim 16, wherein said ramp signal generating circuit comprises: a) a charging circuit comprising a voltage controlled current source and a first capacitor, wherein an input voltage of said voltage controlled current source comprises a difference between said drain-source voltage signal and said output voltage signal, and wherein an output current of said voltage controlled current source is in direct proportion to said input voltage; and b) a first switch, wherein said clock signal controls a switching operation of said first switch to control charging and discharging operation of said charging circuit, wherein a voltage of said first capacitor is configured as said first control signal.
 18. The method of claim 17, further comprising controlling, by a delayed version of said clock signal, said switching operation of said first switch.
 19. The method of claim 16, wherein said comparison circuit comprises a first comparator for receiving said ramp signal and said threshold value, and for generating said comparison signal.
 20. The method of claim 15, wherein said clock signal generating circuit comprises a second comparator for receiving said output voltage signal and said drain-source voltage signal, and for generating said clock signal. 